In the Bayer process, bauxite ore is digested in a caustic slurry to solubilize the alumina values in the ore. The insoluble material (red mud) is then removed by sedimentation in thickeners, which results in an alumina rich liquor which is then seeded to produce the desired alumina trihydrate. Recovered alumina trihydrate can be calcined to an alumina suitable for smelting.
To obtain the maximum recovery of aluminum values from Bayer process liquor, a plant will typically attempt to maximize the liquor caustic level and minimize the final precipitator temperature. However, it is known in the art that a Bayer liquor contaminant, sodium oxalate, can co-precipitate with alumina trihydrate, and that the solubility of sodium oxalate in Bayer process liquor is generally decreased by increasing caustic levels and decreasing precipitation temperatures. Uncontrolled co-precipitation of sodium oxalate can cause a number of problems for a Bayer plant, which thereby places restrictions on plant operating parameters and therefore on alumina values recovery.
Well-known adverse effects of co-precipitation of sodium oxalate include increasing product fines and soda levels, and interference with the efficient size classification of alumina trihydrate. High quality smelting grade calcined alumina usually has a crystal coarseness specification of 90-95% of the crystals being at least 44 microns in diameter. It is known in the art that co-precipitating sodium oxalate can cause the alumina trihydrate crystals to precipitate as a very fine material, which is below the usual quality specification for smelting grade calcined alumina of 90-95% of the crystals being 44 microns or greater in diameter.
Alumina trihydrate is precipitated from Bayer process liquor, which in essence is a concentrated sodium hydroxide solution. Typical soda levels in smelter grade calcined alumina are only 0.2-0.6% by weight as Na.sub.2 O. Nonetheless, it is desirable to produce alumina with soda levels at the lower end of this range. Modern smelters generally calculate their break-even point for soda in alumina at 0.35% Na.sub.2 O.
Input of sodium oxalate into the Bayer process is known in the art to be mainly associated with the digestion of fresh bauxite. Oxalate may be present either as an impurity in the bauxite, or is produced from the caustic degradation of other organic impurities present in the bauxite. To maintain an appropriate steady state sodium oxalate concentration, alumina trihydrate refineries must balance the fresh input of sodium oxalate with a means for its removal from the process. Two methods are widely employed.
In the first oxalate removal method, hereinafter referred to as sidestream oxalate crystallization, sodium oxalate is prevented from co-precipitating with the alumina trihydrate by appropriate control of process conditions. After precipitation and recovery of the alumina trihydrate, the spent liquor is for example partially evaporated, then pumped to a vessel containing sodium oxalate seed causing additional sodium oxalate to precipitate from the spent liquor. Recovered sodium oxalate is used partly as seed for fresh spent liquor and the remainder is removed from the process and destroyed, for example by calcination.
In plants employing the second oxalate removal method, hereinafter referred to as seed washing, the sodium oxalate is allowed to co-precipitate with the alumina trihydrate. The sodium oxalate and finer crystals of alumina trihydrate are then jointly recovered through classification. The sodium oxalate is next removed from the fine alumina trihydrate, generally by hot water washing, and the purified alumina trihydrate is then returned to the process as seed. For Bayer plants using the seed washing method, particularly for those refineries producing a sandy (coarse) alumina, the precipitation of alumina trihydrate can typically be arranged into at least two sequential sections, agglomeration followed by growth. The process conditions are carefully controlled such that co-precipitation of sodium oxalate usually occurs only in the growth section.
A method for inhibiting the precipitation of sodium oxalate in the Bayer process has been recently described in U.S. Pat. No. 5,385,586, wherein certain quaternary amines are added to the liquor prior to or during the alumina trihydrate precipitation step. This method is effective and practical for plants employing the sidestream oxalate crystallization method of oxalate removal because the oxalate stabilizing effect of the additive can be eliminated prior to oxalate crystallization by the heating required for partial liquor evaporation, as shown by Farquharson et. al., Light Metals, 95-101 (1995). By contrast, for plants employing the seed washing method of oxalate removal, no such heating step prior to oxalate co-precipitation is available, and application of quaternary amine liquor oxalate stabilizers to these plants will in principle only lead to an undesirable increase in spent liquor oxalate levels.
Therefore, the control of oxalate co-precipitation in plants employing the seed washing method of oxalate removal is highly desirable. It is also highly desirable to prevent oxalate co-precipitation in agglomeration tanks. Furthermore, it is highly desirable to eliminate oxalate co-precipitation in certain sections of the precipitation circuit because it often leads to production limiting problems in the classification of alumina trihydrate due to the inconsistent formation of poorly settling oxalate-alumina trihydrate morphologies.